The present invention relates to a lift-controlled valve.
The valve seat of the fuel metering device of modem valve-controlled fuel injection systems, diesel injection systems in particular, is exposed to a very high thermal load. Opening the valve completes the injection and the fuel under high pressure is diverted into the return flow via the open valve seat. The pressure energy of the fuel is for the most part converted into thermal energy. This results in the fuel and the surrounding components being very severely heated. Severe thermal expansions of the components resulting from this change the working clearances of the moved components to a corresponding degree. At the same time, the leakage characteristics are changed and accordingly the entire function of the injection system. In an extreme case, the working clearance between the moved components may be reduced to zero. The consequence is jamming or wear in the form of welding of the moved components, resulting in a complete failure of the injection system.
Known high-pressure valves of diesel injection systems have a low-pressure equalizing piston located in the low-pressure area in the cutoff flow direction downstream from the valve seat, the purpose of the low-pressure equalizing piston being to avoid pressure surges on the bottom of the valve needle which occur during valve switching operations.
Such undesirable pressure surges would otherwise bring about a malfunction of the valve needle movement caused by undefined forces. In known valves of the type under discussion, the low-pressure equalizing piston forms an annular gap between the valve needle and the valve body, the angular gap developing a permanent, unchangeable throttling effect, as a result of which a consistent quantity of fuel is withdrawn from the injection system.
The overflow quantity flowing off through the annular gap is continuously replaced by fuel flowing back into the cutoff area (low-pressure area), the fuel thus cooling the high-pressure and filling area of the injection system. The fuel permanently withdrawn via the annular gap flows back into the fuel tank via the return flow.
The object of the present invention is to improve the cooling effect while retaining the total overflow quantity.
According to the present invention, the object is achieved in a lift-controlled valve.
The present invention is based on the idea of draining an increased fuel quantity from the cutoff area into the return flow via the annular gap under discussion when, and only when, the fuel in the cutoff area has been heated to a maximum. This is the case immediately after the opening of the valve seat and the associated cutoff of the fuel under high pressure. This results in an improved cooling of the filling area and cutoff area and a simultaneous increase in the efficiency of the entire injection system.
In addition, the improved cooling reduces the introduction of heat into the components of the valve and thus minimizes the thermal expansion of the components. As a result it is accordingly possible to improve the functional reliability since the working clearances of the moved components of the valve remain more dimensionally stable in operation.